Rapid inbreeding depression in alfalfa is due to loss of intraallelic interactions in tri- and tetraallelic plants. These plants could be identified by using a S1 progeny test and then combined into a higher yielding synthetic variety. The objective of this study was to evaluate the usefulness of S1 progeny test to identify tri and tetraallelic genotypes. Three alfalfa synthetic experimental populations (PSE) were developed by applying three selection methods to an original plant population (PO). The first one, selected those plants whose S1 progenies exhibited higher inbreeding depression (≥65%) on forage production; the second one, selected the plants of the PO that did not produce S1 seed; the last one, consisted on traditional phenotypic selection of PO plants with higher forage yield (15% superior). The elite were manually intercrossed and harvested to produce PSE 1, 2 and 3, respectively. Accumulated forage yield was analyzed for each PSE and PO during the season. All the PSE produced more (p<0.05) than the PO; however, PSE 1 was no different from PSE 3. Data were also used to estimate variance components. The heritability (H) reached a value of 0.86.